Schottky barrier rectifiers are used extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications, such as motor drives, for carrying large forward currents and supporting reverse blocking voltages of up to 100 Volts. Schottky barrier rectifiers are also applicable to a wide range of other applications such as those illustrated in FIG. 1. As is well known to those having skill in the art, rectifiers exhibit low resistance to current flow in a forward direction and a very high resistance to current flow in a reverse direction. As is also well known to those having skill in the art, a Schottky barrier rectifier produces rectification as a result of nonlinear unipolar current transport across a metal-semiconductor contact.
There are basically four distinct processes for the transport of predominantly unipolar charge carriers across a metal/N-type semiconductor contact. The four processes are (1) transport of electrons from the semiconductor over a metal/semiconductor potential barrier and into the metal (thermionic emission), (2) quantum-mechanical tunneling (field emission) of electrons through the barrier, (3) recombination in the space-charge region and (4) hole injection from the metal to the semiconductor. In addition, edge leakage currents, caused by high electric fields at the metal contact periphery, as well as interface currents, caused by the presence of traps at the metal-semiconductor interface, may also be present.
Current flow by means of thermionic emission (1) is generally the dominant process for Schottky power rectifiers with moderately doped semiconductor regions (e.g., Si with doping concentration .ltoreq.1.times.10.sup.16 cm.sup.-3), operated at moderate temperatures (e.g., 300K). Moderate doping of the semiconductor region also generally produces a relatively wide potential barrier between the metal and semiconductor regions and thereby limits the proportion of current caused by tunneling (2). Space-charge recombination current (3) is similar to that observed in a P-N junction diode and is generally significant only at very low forward current densities. Finally, current transport due to minority carrier injection (4) is generally significant only at large forward current densities.
As the voltages of modern power supplies continue to decrease in response to need for reduced power consumption and increased energy efficiency, it becomes more advantageous to decrease the on-state voltage drop across a power rectifier, while still maintaining high forward-biased current density levels. As well known to those skilled in the art, the on-state voltage drop is generally dependent on the forward voltage drop across the metal/semiconductor junction and the series resistance of the semiconductor region and cathode contact.
The need for reduced power consumption also generally requires minimizing the reverse-biased leakage current. The reverse-biased leakage current is the current in the rectifier during a reverse-biased blocking mode of operation. To sustain high reverse-biased blocking voltages and minimize reverse-biased leakage currents, the semiconductor portion of the rectifier is typically lightly doped and made relatively thick so that the reverse-biased electric field at the metal/semiconductor interface does not become excessive. The magnitude of the reverse-biased leakage current for a given reverse-biased voltage is also inversely dependent on the Schottky barrier height (potential barrier) between the metal and semiconductor regions. Accordingly, to achieve reduced power consumption, both the forward-biased voltage drop and reverse-biased leakage current should be minimized and the reverse blocking voltage should be maximized.
Unfortunately, there is a tradeoff between the forward-biased voltage drop and the reverse-biased leakage current in a Schottky barrier rectifier, so that it is generally difficult to minimize both characteristics simultaneously. In general, as the Schottky barrier height is reduced, the forward voltage drop decreases but the reverse-biased leakage current increases. Conversely, as the barrier height is increased, the forward voltage drop increases but the leakage current decreases. The doping level in the semiconductor region also plays a significant role. Generally, the higher the doping level, the lower the forward-biased voltage drop but the reverse-biased breakdown voltage is reduced because of impact-ionization.
Therefore, in designing Schottky barrier rectifiers, design parameters such as barrier heights and semiconductor doping levels are generally selected to meet the requirements of a particular application because all device parasitics cannot be simultaneously minimized. Low barrier heights are typically used for Schottky rectifiers intended for high current operation with large duty cycles, where the power losses during forward conduction are dominant. High barrier heights are typically used for Schottky rectifiers intended for applications with higher ambient temperatures or requiring high reverse blocking capability.
The height of the Schottky barrier formed by the metal/semiconductor junction is related to the work function potential difference between the metal contact and the semiconductor substrate. A graphical illustration of the relationship between metal work function and Schottky barrier height may be found in Chapter 5, FIG. 3 of the textbook by S. M. Sze entitled Semiconductor Devices, Physics and Technology, John Wiley & Sons, 1985, at page 163. A detailed and comprehensive discussion of the design of Schottky barrier power rectifiers may be found in Chapter 4 of a textbook by B. J. Baliga entitled Power Semiconductor Devices, PWS Publishing Co., ISBN 0-534-94098-6 (1995), the disclosure of which is hereby incorporated herein by reference. In particular, sections 4.1.2 and 4.1.3 of the Baliga textbook disclose the semiconductor physics associated with both forward conduction and reverse blocking in a parallel-plane Schottky rectifier, having the structure of FIG. 4.5 therein. As set forth in Equation 4.7, the forward voltage drop is dependent on the drift region, substrate and contact resistances (R.sub.D, R.sub.S and R.sub.C) and the forward current density (J.sub.F), as well as the saturation current (J.sub.S) which is a function of the Schottky barrier height (.phi..sub.bn). The maximum reverse blocking voltage (i.e., breakdown voltage) of a Schottky rectifier (BV.sub.pp) is also disclosed as ideally being equal to that of a one-sided abrupt parallel-plane P-N junction rectifier (e.g., P.sup.+ -N or N.sup.+ -P), having the structure of FIG. 3.3 of the Baliga textbook. The breakdown voltage (BV.sub.pp) is dependent on the doping concentration of the drift region (N.sub.D), as described by Equation (1) below. EQU N.sub.D =2.times.10.sup.18 (BV.sub.pp).sup.-4/3 ( 1)
Equation (1) is a reproduction of Equation 4.11 from the aforementioned Baliga textbook. A graphical representation of breakdown voltage and depletion layer width (W.sub.pp) at breakdown versus drift region doping (N.sub.D) for an abrupt parallel-plane P-N junction rectifier is shown by FIG. 2. FIG. 2 is a reproduction of FIG. 3.4 from the aforementioned Baliga textbook.
In reality, however, the actual breakdown voltage of a conventional Schottky rectifier is about one-third (1/3) that for the abrupt parallel-plane P-N junction rectifier described by Equation (1) and graphically illustrated by FIG. 2. As will be understood by those skilled in the art, the reduction in breakdown voltage below the theoretical ideal parallel plane value is caused, in part, by image-force-induced lowering of the potential barrier between the metal and the semiconductor regions, which occurs at reverse-biased conditions.
One attempt to optimize the on-state voltage drop/reverse blocking voltage tradeoff associated with the Schottky barrier rectifier is the Junction Barrier controlled Schottky (JBS) rectifier. The JBS rectifier is a Schottky rectifier having an array of Schottky contacts at the face of a semiconductor substrate with corresponding semiconductor channel regions beneath the contacts. The JBS rectifier also includes a P-N junction grid interspersed between the Schottky contacts. This device is also referred to as a "pinch" rectifier, based on the operation of the P-N junction grid. The P-N junction grid is designed so that the depletion layers extending from the grid into the substrate will not pinch-off the channel regions to forward-biased currents, but will pinch-off the channel regions to reverse-biased leakage currents.
As will be understood by those skilled in the art, under reverse bias conditions, the depletion layers formed at the P-N junctions spread into the channel regions, beneath the Schottky barrier contacts. The dimensions of the grid and doping levels of the P-type regions are generally designed so that the depletion layers intersect under the array of Schottky contacts, when the reverse bias exceeds a few volts, and cause pinch-off. Pinch-off of the channels by the depletion layers cause the formation of a potential barrier in the substrate so that further increases in the reverse-biased voltage are supported by the depletion layers. Accordingly, once a threshold reverse-biased voltage is achieved, the depletion layers shield the Schottky barrier contacts from further increases in the reverse-biased voltage. This shielding effect generally prevents the lowering of the Schottky barrier potential at the interface between the metal contacts and semiconductor substrate and inhibits the formation of large reverse leakage currents.
The design and operation of the JBS rectifier is described in Section 4.3 of the aforementioned Baliga textbook and in U.S. Pat. No. 4,641,174 to Baliga, entitled Pinch Rectifier, the disclosure of which is hereby incorporated herein by reference. For example, as shown by FIG. 6 of the '174 patent, reproduced herein as FIG. 3, an embodiment of a pinch rectifier 200 comprises a plurality of Schottky rectifying contacts 232 formed by metal layer 230 and substrate 204 and a P-N junction grid formed by regions 234 and substrate 204. Unfortunately, the JBS rectifier typically possesses a relatively large series resistance and a relatively large on-state forward voltage drop caused by the reduction in overall Schottky contact area dedicated to forward conduction. This reduction in area is necessarily caused by the presence of the P-N junction grid which occupies a significant percentage of the total area at the face of the substrate. In addition, large forward currents can cause large forward voltage drops and can lead to the onset of minority carrier conduction (i.e., bipolar), which limits the performance of the JBS rectifier at high switching rates. Finally, although the reverse blocking voltage for the JBS may be somewhat higher than the reverse blocking voltage for a Schottky rectifier having an equivalent drift region doping (N.sub.D), it generally does not achieve the level of reverse blocking capability attainable with a parallel-plane P-N junction, as illustrated by FIG. 2.
Another attempt to optimize the forward voltage drop/reverse blocking voltage tradeoff is disclosed in U.S. Pat. No. 4,982,260 to Chang et al. entitled Power Rectifier with Trenches, the disclosure of which is hereby incorporated herein by reference. For example, as shown by FIGS. 10B and 14B, reproduced herein as FIGS. 4 and 5, respectively, conventional P-i-N rectifiers (P.sup.+ -N.sup.- -N.sup.+) are modified to include an interspersed array of Schottky contacts on a face of an N-type semiconductor substrate. As shown by FIG. 4, the Schottky contact regions 550A-C are separated from the P.sup.+ portions 510A-D (of the P-i-N rectifier) by MOS trench regions 522A-522F. In another embodiment shown by FIG. 5, the Schottky contact regions 718A-E are interspersed adjacent the P.sup.+ portions 720A-F, which are formed at the bottom of trenches 710A-F. As will be understood by those skilled in the art, these modified P-i-N rectifiers also typically possess an unnecessarily large series resistance in the drift region (N.sup.- regions 506, 706). Moreover, only a relatively small percentage of forward-conduction area is dedicated to the Schottky contacts, which dominate the forward bias characteristics by turning on at lower forward voltages than the parallel connected P.sup.+ -N junctions. Finally, although the forward leakage current for these P-i-N type rectifiers is substantially lower than the corresponding forward leakage current for a Schottky rectifier, like the JBS rectifier, they do not achieve the level of reverse blocking capability associated with an abrupt parallel-plane P-N junction.
However, U.S. Pat. No. 5,365,102 to Mehrotra and inventor Baliga, entitled Schottky Barrier Rectifier with MOS Trench, the disclosure of which is hereby incorporated herein by reference, discloses Schottky barrier rectifiers which have a higher breakdown voltage than theoretically attainable with an ideal abrupt parallel-plane P-N junction. A cross-sectional representation of one embodiment of the described rectifiers, referred to as the TMBS rectifier, is illustrated by FIG. 6 and described in an article by Mehrotra and inventor Baliga entitled Trench MOS Barrier Schottky (TMBS) Rectifier: A Schottky Rectifier With Higher Than Parallel Plane Breakdown Voltage, Solid-State Elec., Vol. 38, No. 4, pp. 801-806 (1995), the disclosure of which is hereby incorporated herein by reference.
In particular, better than theoretically ideal breakdown voltage characteristics were achieved because of the occurrence of charge coupling between the majority charge carriers in the mesa-shaped portion of the epitaxial/drift region and the metal on the insulated sidewalls of the trenches. This charge coupling produced a redistribution of the electric field profile under the Schottky contact which resulted in a breakdown voltage of about 25 Volts being achieved for a uniform drift region doping concentration of 1.times.10.sup.17 cm.sup.-3 and oxide thickness of 500 .ANG., as opposed to 9.5 Volts for an ideal abrupt parallel-plane rectifier. Furthermore, because the peak electric field at the metal-semiconductor contact was reduced relative to an ideal rectifier, reverse leakage current was also reduced.
The redistribution of the electric field profile, relative to an ideal parallel-plane rectifier with drift region doping concentration of 3.times.10.sup.16 cm.sup.-3, is illustrated by FIG. 7 for various trench depths ("d"). FIG. 7 is a reproduction of FIG. 2 from the aforementioned Mehrotra and Baliga article. As shown by FIG. 7, there are at least two distinct effects associated with the charge coupling between the trench electrodes and mesa. First, the electric field at center of the Schottky contact is reduced significantly and second, the peak in the electric field profile shifts away from the metal-semiconductor contact and into the drift region. The reduction in electric field at the center of the Schottky contact causes a significant decrease in the reverse leakage current through a reduction in Schottky barrier height lowering and as the peak of the electric field moves away from the Schottky contact, the mesa is able to support more voltage than parallel-plane theory predicts.
A graphical illustration of breakdown voltage versus trench oxide thickness for the TMBS rectifier of FIG. 6 is illustrated by FIG. 8, which is a reproduction of FIG. 4(b) from the aforementioned Mehrotra and Baliga article. As shown by FIG. 8, increases in oxide thickness beyond 750 .ANG. cause a significant decrease in breakdown voltage. This decrease in breakdown voltage with increasing oxide thickness can be attributed to reduced charge coupling between the anode electrode on the trench sidewalls and the mesa-shaped portion of the drift region. A graphical illustration of breakdown voltage versus trench depth for the TMBS rectifier of FIG. 6 is also illustrated by FIG. 9, which is a reproduction of FIG. 3 from the aforementioned Mehrotra and Baliga article. As shown by FIG. 9, increases in trench depth beyond 2.5 .mu.m do not cause a continuing increase in breakdown voltage beyond 25 Volts.
Notwithstanding these developments, however, there continues to be a need for even more efficient rectifiers which are capable of blocking even higher voltages with less reverse leakage current than the aforementioned devices, including the TMBS rectifier.